 | Acta Crystallographica Section D Acta Crystallographica Section D BIOLOGICAL CRYSTALLOGRAPHY |
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![[Figure 2]](ad0185fig2mag.jpg)
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Figure 2
Colour code: red helix, αN of histone H3; orange histone-fold pair, H2A′–H2B′; Yellow histone-fold pair, H3′–H4′; red histone-fold pair, H3–H4; green histone-fold pair, H2A–H2B; long (red) loop, LH3; magenta, αC helix of histones H2B; black spheres. water molecules; green spheres, chloride ions. (a) The histone octamer in KCl/phosphate viewed with the pseudo-twofold axis vertical and the axis of would-be DNA coiling horizontally. The labels starting with a `T' correspond to the last amino-acid residue in the N-terminal tail of the appropriate histones. The label CT H2A indicates the first residue in the C-terminal tails of histones H2A. These residues are next in sequence to α helices in the `structured' part of the octamer, which lock them in position. LH3 is the loop which allows the αN helix of histone H3 to be `switched' to the right of the figure. (b) An enlarged section of Fig. 2 ![[link]](../../../../../../logos/arrows/d_arr.gif) (a), showing the three ways that the histone-pair (H2A′–H2B′) docks against the (H3–H4) histones in forming the helical histone track. The square box shows the so-called molecular cluster formed by hydrogen bonding between the H2A′ docking sequence (residues Ala103–Leu115), the αN helix of H3 (residues Gly55 and Thr58), the α2 helix of H3 (residue Glu94) and part of the L1 loop of histone H4 (residue Lys44). See also Fig. 2(c). This molecular cluster stabilizes the αN helix of H3, to control DNA-entry and DNA-exit binding, and locates the starting points of the N-terminal tail of H3 and the C-terminal tail of H2A′. Interactions between regions called docking sequences (see also Figs. 1 and 5c) occur as a result of hydrogen bonds between residues Arg95–Tyr98 of H4 and residues Lys99–Thr103 of H2A′. These interactions form a three amino-acid parallel β-sheet (Fig. 1). Residues Tyr98–Gly102 of the H4 docking sequence form hydrogen bonds with the α2 histone-fold helix of histone H2B′ (residues Ser64 and Asp68). There are numerous non-covalent interactions which link the H2A′ docking sequence and the H4 docking sequence to their own side of the helical track (see §1). Similar cross-ties also occur for the H2A and H4′ docking sequences. (c) A stereo figure showing how the position of the αN helix of histone H3′, relative to the (H2A. H2B) histone pair, is determined by the H2A docking sequence (residues Ala103–Leu115 of H2A), which forms a molecular cluster with this αN helix (residues Gly55 and Thr58 of H3′), the α2 helix of H3′ (residue Glu94) and part of the L1 loop of histone H4′ (residue Lys44). The clustering is via many hydrogen bonds, both direct and via waters. Chloride ion t is also shown. |
![[Figure 2]](ad0185fig2.jpg)
| BIOLOGICAL CRYSTALLOGRAPHY |
ISSN: 1399-0047
